1. Field of the Invention
The present invention relates to an alignment method of plural diffraction gratings, and the diffraction gratings that are easy to align, in a radiation imaging system for capturing a phase contrast image of an object with use of the diffraction gratings.
2. Description Related to the Prior Art
X-rays are used as a probe for imaging inside of an object without incision, due to the characteristic that attenuation of the X-rays depends on the atomic number of an element constituting the object and the density and thickness of the object. Radiography using the X-rays is widely available in fields of medical diagnosis, nondestructive inspection, and the like.
In a conventional X-ray imaging system for capturing a radiographic image of the object, the object to be examined is disposed between an X-ray source for emitting the X-rays and an X-ray image detector for detecting the X-rays. The X-rays emitted from the X-ray source are attenuated (absorbed) in accordance with the characteristics (atomic number, density, and thickness) of material of the object present in an X-ray path, and are then incident upon pixels of the X-ray image detector. Thus, the X-ray image detector detects an X-ray absorption image of the object. There are some types of X-ray image detectors in widespread use, such as a combination of an X-ray intensifying screen and a film, an imaging plate containing photostimulated phosphor, and a flat panel detector (FPD) that takes advantage of semiconductor circuits.
The smaller the atomic number of the element constituted of the material, the lower X-ray absorptivity the material has. Thus, the X-ray absorption image of living soft tissue, soft material, or the like cannot have sufficient contrast. Taking a case of an arthrosis of a human body as an example, both of cartilage and joint fluid surrounding the cartilage have water as a predominant ingredient, and little difference in the X-ray absorptivity therebetween. Thus, the X-ray absorption image of the arthrosis hardly has sufficient contrast.
With this problem as a backdrop, X-ray phase imaging is actively researched in recent years. In the X-ray phase imaging, an image (hereinafter called phase contrast image) is obtained based on phase shifts (shifts in angle) of the X-rays that have passed through the object, instead of intensity distribution of the X-rays having passed therethrough. It is generally known that when the X-rays are incident upon the object, the phases of the X-rays interact with the material more closely than the intensity of the X-rays. Accordingly, the X-ray phase imaging, which takes advantage of phase difference, allows obtainment of the image with high contrast, even in capturing the image of the object constituted of the materials that have little difference in the X-ray absorptivity. As a type of such X-ray phase imaging, is proposed an X-ray imaging system using an X-ray Talbot interferometer, which is constituted of two transmission diffraction gratings and the X-ray image detector (refer to Japanese Patent Laid-Open Publication No. 2008-200359 and Applied Physics Letters, Vol. 81, No. 17, page 3287, written on October 2002 by C. David et al., for example).
The X-ray Talbot interferometer is constituted of the X-ray source, the X-ray image detector, and first and second diffraction gratings disposed between the X-ray source and the X-ray image detector. The first diffraction grating is disposed behind the object. The second diffraction grating is disposed downstream from the first diffraction grating by a specific distance (Talbot distance), which is determined from a grating pitch of the first diffraction grating and the wavelength of the X-rays. The Talbot distance is a distance at which the X-rays that have passed through the first diffraction grating form a self image by the Talbot effect. If the object is disposed between the X-ray source and the first diffraction grating or between the first diffraction grating and a self image observation position (in this case, the Talbot distance), this self image is spatially modulated according to the interaction (phase shifts) between the X-rays and the object.
In the X-ray Talbot interferometer, the second diffraction grating is overlaid on the self image of the first diffraction grating to obtain a fringe image subjected to intensity modulation. The fringe image is detected by a fringe scanning technique. From variation (phase shift) of the fringe image due to the presence of the object, an intensity contrast image of the object is obtained. In the fringe scanning technique, a plurality of images are captured, while the second diffraction grating is slid relatively against the first diffraction grating in a direction substantially parallel to a surface of the first diffraction grating and substantially orthogonal to a grating direction of the first diffraction grating at a scan pitch that corresponds with an equally divided part of a grating pitch. By this scanning operation, is obtained series data (hereinafter called intensity modulation signal) composed of pixel data the intensity of which is periodically changed on a pixel basis of the X-ray image detector. From a phase shift amount (a phase shift amount between the presence and the absence of the object) of this intensity modulation signal, a differential phase image (corresponding to angular distribution of the X-rays refracted by the object) is obtained. Furthermore, integration of the differential phase image along a fringe scanning direction allows obtainment of the phase contrast image. This fringe scanning technique is also adopted in an imaging system using laser light (refer to Applied Optics, Vol. 37, No. 26, page 6227, written on September 1998 by Hector Canabal et al.).
If the focus size of the X-ray source is large, the self image of the first diffraction grating is blurred, and the blur causes degradation in image quality of the phase contrast image. To prevent this problem, is known an X-ray imaging system having a third diffraction grating (source grating) disposed just behind the X-ray source. The third diffraction grating partly blocks the X-rays emitted from the X-ray source to reduce the effective focus size, and forms a number of narrow point sources (distributed light sources), in order to prevent the blur of the self image.
To capture the phase contrast image of high image quality, it is necessary to precisely align the X-ray source, the first to third diffraction gratings, and the X-ray image detector. Especially, it is expected that a relative rotation angle among the first to third diffraction gratings is difficult to adjust because slight deviation in the relative rotation angle severely affects the image quality of the phase contrast image. In the X-ray imaging system used in a laboratory, the precise alignment is carried out through use of a measuring instrument with high degree of accuracy. However, in the commercial X-ray imaging system set up in a hospital or the like, the use of the highly accurate measuring instrument is out of reality, and the establishment of any alignment technique is desired.